Seat back



FIG. 1 is a front perspective view of a seat back forming according to my new design, a chair being shown in broken lines for environmental purposes only and form no part of the claimed design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is a left side elevational view thereof, the right side being a mirror image; and,

FIG. 5 is a top plan view thereof. 

The ornamental design for a seat back, as shown and described. 